1. Field of the Invention
The present invention relates to relates to thermally managed electrical energy storage device enclosures and self-managing thermal enclosures.
2. The Prior Art
With the increased market penetration of all-electric and hybrid electric vehicles, a variety of new electrical energy storage devices are being developed and deployed in the vehicle industry.
Such electrical energy storage devices include, for example electrochemical electrical energy storage devices (such as, for example, batteries or cells) and capacitive-type energy storage devices (such as, for example, ultracapacitors).
Although many of these electrical energy storage devices offer improved performance (resulting in longer life and vehicle range), they still suffer from thermal management challenges. In many cases these challenges are amplified by increased energy requirements and increased power density.
Based on the electrical losses of any electrical circuit (specifically “IR” or current and resistance loses), the electrical energy storage system is subject to a high degree of current density (amps per area w/m2) and therefore heating. Heating causes a number of problems for any electrical energy storage device. In particular, heating causes the following three problems in electrical energy storage devices:
(1) thermal cycling which causes dimensional instability (swelling) and results in degradation of the electrical energy storage device enclosure and an increase of the internal electrical conductive paths of the electrical energy storage device, which further increases the internal resistance (and heating) and results in a lower operating voltage;
(2) chemical breakdown of the internal electrical energy storage device structure from elevated temperatures; and
(3) thermal runaway (uncontrolled heating).
Electrical energy storage device usage patterns, especially for large demand applications such as for electric vehicles, generate significant heat during rapid charge and discharge cycles. At such points current levels may exceed the device's rating, generating even more heat.
From a practical perspective, as electrical energy storage device size increases, the ratio of heat/cooling surface area to power generating volume typically decreases. Thus, as variations in charge or discharge current increases, so does the subsequent amount of generated heat. As a result, the temperature of the electrical energy storage device rises dramatically.
The availability of discharge power, available energy, and sometimes even charge acceptance are all influenced by electrical energy storage device temperature affects. Ideally, electrical energy storage devices will operate in a temperature range that optimizes electrical energy storage device performance and life. However, practically speaking, even temperature variation between modules in a pack of electrical energy storage devices causes differences in the performance of each module leading to an unbalanced pack and reduced pack performance.
Electrical energy storage device thermal management systems seek to minimize temperature variation between electrical energy storage devices and to keep the electrical energy storage devices closer to the ideal operating temperature range. Thermal management designs are best if they are lightweight and compact.
Prior art thermal management systems for electrical energy storage devices typically make use of air, liquids, insulation, thermal storage or phase change materials. Thermal management designs which use power to perform the thermal management increase the electrical energy storage device capacity that must be carried and can increase the heat generation problem even further.
The heat generated from the electrical energy storage device under load conditions typically influences the type and size of the cooling system required. The heat generation is due to both electrochemical enthalpy change as well as electrical resistive heating. The rate of discharge, and hence heat generated, depends on the chemistry type, construction, temperature, state of charge, and discharge or charge profile. Heat generation is temperature dependent and in general, more heat is generated at lower temperatures due to the increased resistance in the electrical energy storage device.
Prior art thermal management system designs include a number of physical interfaces with the electrical energy storage device. For example, in prior art designs, the electrical energy storage device may be surrounded by an isolation wrapping, a conductive bridge and an enclosure body to which a heat sink is attached. Each thermal interface boundary localizes a buildup of stored heat, resulting in disadvantageous reliability and performance issues.
Accordingly, there is a need for a thermally managed electrical energy storage device enclosure which minimizes thermal interface boundaries, such as for example, isolation wrapping, conductive bridges and heat sinks.
A need further exists for a thermally managed electrical energy storage device enclosure which exerts a uniform radial force on the electrical energy storage device or devices, thereby providing shock, vibration, and abrasion protection and thermal conductivity.
A need further exists for a thermally managed electrical energy storage device enclosure wherein the structure of the enclosure acts as a thermal management system which provides enhanced thermal conductivity, low overall manufacturing cost, weight reduction and volume reduction.